A technique disclosed in the present invention relates to semiconductor elements as terahertz wave radiating elements capable of radiating electromagnetic waves in a terahertz frequency band.
Electromagnetic waves in the terahertz band show transparency of radio waves as well as the broadband property, coherence, and concentrating property of light, and have been expected to be applied to various fields such as security, biology, environment, and communication. Photoconductive elements, terahertz parametric generators, quantum cascade lasers, carbon dioxide lasers, backward wave oscillators, etc. have been used as devices for radiating terahertz electromagnetic waves. However, currently, it is difficult to reduce the size and prices of such devices because they require a large-scale, expensive femtosecond laser or picosecond laser, and need to be cooled to a very low temperature when operated, etc.
Devices using electron plasma that is present in semiconductors have been intensively studied and developed in order to implement small, inexpensive terahertz wave radiating devices that need not to be cooled. A two-dimensional electron gas (2DEG) in field effect transistors (FETs) or high electron mobility transistors (HEMTs) is used as the electron plasma in the semiconductors. The 2DEG plasma waves as collective oscillations of electrons have a velocity on the order of 106 m/s, which is at least one digit higher than the drift velocity of a single electron. Thus, operation in the terahertz frequency band, which is difficult to implement in common electronic devices using a single electron, can be implemented by using the plasma waves.
FIG. 8 shows a cross-sectional structure of a conventional terahertz wave radiating element (see, e.g., FIG. 1 of Physical Review Letters, Vol. 71, No. 15, pp. 2465-2468 (1993)). As shown in FIG. 8, the conventional terahertz wave radiating element is formed by a semiconductor layer 4, a gate insulating film 5, a two-dimensional electron gas (2DEG) region 6, a source electrode 1, a gate electrode 2, and a drain electrode 3. If a bias voltage is applied between the source electrode 1 and the drain electrode 3 in the terahertz wave radiating element having such a configuration, a 2DEG flow is produced in the 2DEG region 6. The 2DEG flow becomes an unstable state when it is reflected at the ends of the source and drain electrodes 1, 3 (plasma instability). In recent FET structures with reduced gate lengths, this plasma instability induces electron plasma oscillations in the terahertz frequency band, causing radiation of terahertz electromagnetic waves.
Recently, significant progress has been made in this technique, and it was verified in 2006 that terahertz electromagnetic waves are radiated at room temperature (see, e.g., Applied Physics Letters, 88, 141906 (2006)). According to this document, a terahertz electromagnetic wave output of about 0.1 μW is obtained from AlGaN/GaN HEMTs.
Terahertz wave radiating elements having a gate electrode formed in a grid pattern on a semiconductor heterostructure have also been proposed (see, e.g., Applied Physics Letters, 89, 263502 (2006), International Publication No. WO 2006/030608, and Japanese Patent Publication No. 2009-224467).
However, such semiconductor devices capable of radiating terahertz waves at room temperature by using electron plasma also have the following problems. The intensity of terahertz electromagnetic waves that can be radiated by such semiconductor devices is as low as 1 μW or less, and thus the output is not high enough for various applications such as security, inspection, etc. At present, power conversion efficiency is 1×10−7, which is extremely low, and input power as high as about 1 W is required to provide an output of 0.1 μW. In order to implement practical terahertz wave radiating semiconductor devices, it is essential to increase the output and the power conversion efficiency.